Field of the Invention
The invention relates to a method and device for driving a capacitive actuator, in particular a piezoelectric-operated fuel injection valve of an internal combustion engine.
Piezo actuators consist of a multiplicity of piezo-ceramic layers that form a so-called "stack." The stack changes its dimensions when a voltage is applied, in particular, it changes its length s by a displacement ds, or it produces an electric voltage on mechanical stress or tension.
Various methods are known for driving piezo actuators, which behave electrically as capacitors. In those methods the voltage applied to the piezo actuator is monitored. In all prior art methods, the criterion used for terminating the charging is when a specific voltage across the piezo actuator is reached, unless an elaborate measurement of the relevant actuator displacement ds is provided. One example of this is a prior German patent application No. 1932872-1.
The electrical properties of a piezo stack of the foregoing type change with the temperature to which it is exposed. With increasing temperature, its capacitance increases, and the displacement increases as well. At temperatures, from about -40.degree. C. to +150.degree. C., which are meant to be taken into account for automotive applications, changes of up to a factor of 2 may in this case be observed.
If a piezo actuator is charged, at all operating points, for example to a constant voltage, which brings about the requisite displacement ds at low temperatures, then the displacement attained at high temperatures is considerably greater than necessary. In the context of fuel injection valves with constant fuel pressure, this leads to an excessive amount of fuel, or vice versa. Since the capacitance of the piezo stack is likewise greater at high temperatures, very much more charge and energy (E=1/2C.multidot.U.sup.2) than necessary is required.
U.S. Pat. No. 5,387,834 discloses a drive circuit for a piezoelectric element of a dot matrix printer, in which a temperature sensor senses the temperature of the piezoelectric element. The piezoelectric element is driven using charging times which are stored in a table as a function of the temperature.
U.S. Pat. No. 5,543,679 discloses a drive circuit for a piezoelectric element for driving a fuel valve. A presumably constant charge is drawn from a capacitor and supplied to the piezoelectric element via a transformer. None of the variations in the circuit due to temperature changes, humidity, component tolerances, aging, and the like are thereby taken into consideration.
Patent Abstracts of Japan, Vol. 018, No. 188 (E-1532), Mar. 31, 1994 & JP-A 05 344755 discloses a drive circuit for a piezoelectric element for driving a fuel valve. There, the piezoelectric element is charged via a first regulated voltage with a constant charge amount, and is entirely discharged to a regulated second negative voltage via a capacitance determination of the piezoelectric element of the voltage measured at discharge.